Olin niin syventynyt kirjaan, etten edes tajunnut auringon olleen laskenut kunnes joku tuli tervehtimään minua ja katsoin ylös.
"I was so absorbed in my book that I didn't even notice that the sun had gone down, until someone came to greet me and I looked up."

Instead of auringon olleen laskenut, you could also say taivaan olleen pimenneet (= "that the sky had darkened").